1. Field of the Invention:
This invention relates generally to bearing members such as piston rings, packing rings, and valves and, more particularly to the bearing faces of such members having a high temperature, oxidation-resistant, wear-resistant and scuff-resistant coating thereon applied by plasma-spraying a particle composition onto such bearing faces.
2. Description of the Prior Art:
In internal combustion engines, each piston usually has a plurality of compression rings and oil control rings in grooves located on the side face of the piston which bear against the cylinder wall of the engine to provide a seal between the piston and the cylinder wall. Failure of the piston rings to properly seal the piston and the cylinder results in leakage between the rings and the cylinder thereby causing low engine compression, bad ignition, incomplete combustion and accelerated erosion of the piston rings and cylinder wall. Because the piston rings reciprocate at high speeds against the cylinder walls at high operating temperatures, the rubbing surfaces tend to wear rapidly. Accordingly, many attempts have been made to develop coatings for the wear surfaces on the piston rings and cylinder walls that can extend their life.
Many flame-sprayable compositions have been developed for coating such wear surfaces. For example, one composition includes commercially pure elemental molybdenum as one constituent of the coating such as shown in Hyde et al. Pat. No. 3,556,747. Such coatings have improved the wear resistance of piston rings that operate at relatively low temperatures but, at elevated operating surface temperatures, the elemental molybdenum apparently oxidizes and forms molybdenum oxides at a threshold temperature of about 450.degree. Fahrenheit. These oxides seem to migrate to the grain boundaries of the molybdenum coating causing the cohesive strength of the coating to decrease and eventually results in flaking of the coating from the piston ring or other wear surfaces to which it is applied.
Other refractory metals such as molybdenum, chromium, and tungsten carbides have been used such as shown in Prasse Pat. No. 3,539,192 and McCormick Pat. No. 3,606,359. While these carbide coatings have made the piston rings more wear resistant, it has been discovered that they are often too abrasive and tend to induce engine cylinder wear and usually have insufficient mechanical strength to withstand the high speed and high temperatures in modern engines. In fact, this problem has been recognized by McCormick in Pat. No. 3,606,359. McCormick states that metal carbide coatings carry sharp-edged or globular particles that pull out of the coating in operation in the engine causing excessive wear of the piston rings and cylinders. McCormick attacked this particular problem by placing the carbides in solution with cobalt. He also believed that if the amount of aluminum in the coating is less than 1% by weight, the problem of "scuffing" would be avoided.
Scuffing occurs when the coating has an adhesive affinity for the wall of the engine cylinder. The coating on the piston ring contacts the bearing face of the adjacent member through minute asperities. Since the areas of actual contact are so small, plastic flow occurs and the two surfaces become cold welded. Relative motion between the surfaces shears the welded junctions and generates loose particles of debris which contribute to the scuffing of the cylinder wall or other adjacent member.
Although coatings comprised of or containing elemental molybdenum are usually scuff-resistant at low temperatures, high frictional surface temperatures in excess of 1000.degree. F are now present in some newer engines; thus, more scuff-resistant and oxidation-resistant coatings are now needed such as provided by the use of more oxidation-resistant carbides and alloys of this invention.
In U.S. Pat. 3,615,099, Prasse proposes the use of multiple layers of flame-sprayed compositions and specifically the use of an outer layer of elemental molybdenum and an inner layer of a more refractory material such as a tungsten carbide alloy. Yet, Prasse recognizes that the outer molybdenum layer will not last long and uses it specifically as a break-in layer that shortly wears away after a new piston ring has been broken in. After the outer molybdenum layer vanishes, the more refractory layer of a tungsten carbide alloy becomes the effective seal.
The coatings described in the foregoing patents generally share one thing in common; they all use an intermetallic alloy as the starting material which serves as a bonding agent in the final coating. To this alloy are added specific amounts of other materials in an attempt to capture the beneficial characteristics of the added materials.
From this it might be theorized that one has only to add those materials possessing the desired characteristics to produce an improved coating.
Unfortunately, the synergistic qualities desired in the coating cannot be predetermined by formulas, rules, or knowledge of the characteristics of the individual materials used. If this could be done, then it would only be necessary to select materials having the desired characteristics and combine them to achieve a coating having all of the desired qualities. Obviously, this cannot be done; otherwise an ultimate coating would have been developed long ago. As it is, only inventive effort succeeds in achieving the desired result because of the complex interaction of various materials when subjected to the intense heat of the plasma flame and only testing proves the results of the inventive effort.
In addition, those working in the prior art have not recognized the importance of preparing the starting material such that substantial alloying of the materials occurs in the final coating, except that it has been recognized that tungsten carbide should be alloyed with cobalt to form one constituent of the starting material.
We have discovered that the wear-resistance and scuff-resistance of a coating can be greatly improved by alloying molybdenum with tungsten carbide, nickel, and aluminum in the final coating, especially for use where surface temperatures in excess of 1000.degree.F occur in an internal combustion engine or other apparatus in which the coating is used and that it improves adhesion of the coating to the underlying ferrous body, improves the cohesive strength of the coating itself, and improves the anti-welding properties of the coating.
We have also discovered that alloying of the materials in the final coating can best be achieved by properly preparing the aggregate to be sprayed. Doing so prevents the aluminum constituent from buring up in the plasma flame, prevents oxidation of and preserves the scuff-resistant properties of the molybdenum, enables the formation of nickel-molybdenum alloys, and permits the tungsten carbide to at least partially if not completely alloy with the other materials in the coating.